The present invention relates to an improved substrate of a magnetic recording medium, referred to simply as a substrate hereinafter, such as a disk prepared, for example, from a wafer of a single crystal of silicon.
Along with the rapid progress of the information-predominant society in recent years, the demand is also rapidly increasing for large-capacity recording media for information used in computers and other electronic data-processing machines. In particular, it is a remarkable trend that magnetic memory disks which play a core role as an external memory unit in computers are required to have a recording capacity and recording density increasing year by year so that intensive and extensive investigations are now under way to develop a magnetic memory disk suitable for high-density recording. Besides the increased recording density mentioned above, the development works on one hand are directed to a magnetic recording disk having improved mechanical strengths in view of the recent progress in very compact portable computers such as the so-called notebook-type or palm-top personal computers of which the recording system naturally must be capable of withstanding mechanical shocks. If not to mention mechanical shocks, magnetic recording disks must have high mechanical strengths to withstand the considerably high velocity of revolution when they are under operation as is mentioned later.
As is known, the most conventional material for the substrates, which is made from a non-magnetic material and has an annular form of a disk with a circular outer contour and provided with an also circular concentric center opening, is an aluminum-based alloy while, as a trend, aluminum alloy-made substrates cannot meet the increasing requirements for a smaller surface roughness to enable an increase in the recording density as well as higher abrasion resistance to ensure longer durability of the magnetic disks. These requirements necessitate the use of a material having an increased hardness. Along with the development of very compact computers, in particular, it is essential that the power consumption for driving of the magnetic disk is as small as possible. In this regard, the substrate must be as light as possible although a large part of the power in magnetic disk driving is consumed in the spindle motor per se. In other words, a substrate material having a smaller specific gravity than aluminum-based alloys is desired.
Needless to say, the coercive force of the magnetic recording layer is an important factor to ensure a high recording density therein and should desirably be as high as possible. It is known that cobalt-based alloys as a class of the conventional materials for the magnetic recording layer have a higher and higher coercive force as the temperature of the film forming is increased up to a certain upper limit of the temperature. In this regard, it is a usual practice that the substrate of a magnetic recording medium is heated during the forming process of the magnetic recording layer by sputtering or other methods. This heating treatment, however, sometimes causes a serious trouble such as warping of the disk when the substrate is made from an aluminum-based alloy because, while it is usual that the surface of an aluminum alloy-made substrate is provided with a plating layer of NiP (nickel phosphide)-in order to avoid the problems of low abrasion resistance and poor machinability due to the low hardness of the aluminum-based alloys, of the considerably large difference in the thermal expansion coefficient between the aluminum-based alloy and the NiP plating layer. Moreover, a plating layer of NiP is rendered magnetic at a temperature of 280.degree. C. or higher so that the temperature of the heating treatment of a NiP-plated substrate is necessarily limited.
Alternatively, glass-made substrates are under use partly in the manufacture of magnetic recording disks with an object to obtain a very small surface roughness not obtained with substrates of an aluminum-based alloy. A problem in such a glass substrate is that, since glass substrate plates are usually subjected to a tempering treatment in order to be imparted with increased mechanical strengths, warping is sometimes caused of a glass substrate when it is subsequently heated due to the presence of stressed surface layers which are under a compressive stress. While it is usual to use an infrared heater to heat up the substrate in the sputtering process in the formation of the coating layers, the power output of the infrared heater must be unduly increased in order to achieve the desired high temperature because glass substrates have a relatively low absorptivity of the infrared energy. Consequently, the forming process of the magnetic recording layer on the surface of a glass substrate is sometimes conducted at a temperature not high enough so that the thus formed magnetic layer has only a relatively low coercive force requiring a large electric power consumption in recording therein.
Further alternatively, substrate plates made from a single crystal of silicon are highlighted in recent years by virtue of the various advantageous properties of silicon single crystals including their good high-temperature performance, small thermal expansion coefficient, smaller specific gravity than aluminum-based alloys and adequate electric conductivity. Namely, a substrate of a silicon single crystal, as compared with a substrate of an aluminum-based alloy, has advantages including: safety from the risk of warping under heating as a consequence of absence of a surface layer of a different material or a stressed layer; and excellent heat resistance to withstand heating at a high temperature of 600.degree. C. or higher. In addition, guide grooves for tracking and other surface structures can be readily formed on a silicon substrate by applying the technology of fine patterning works well established in connection with the manufacturing process of LSIs and other electronic devices.
The manufacturing process of substrates of a silicon single crystal is performed usually in the steps including: cylindrical grinding of a single crystal of silicon in the form of an elongated rod on a cylindrical grinding machine until a cylinder of the single crystal having a diameter equal to that of the desired substrate disks is obtained; boring the single crystal cylinder along the center axis of the cylinder concentrically with the outer surface of the cylinder to form a center bore having a diameter equal to that of the center opening in the annular substrate disk; slicing the thus bored single crystal cylinder in a plane perpendicular to the center axis of the cylinder into annular disks each having a center opening; lapping of the thus sliced annular disk to adjust the thickness and to remove the saw mark; chamfering of the outer periphery of the disk and the inner periphery facing the center opening so as to impart the chamfered peripheries with a trapezoidal cross sectional profile of a bevelled surface; and polishing of the surfaces of the disk so as to impart the disk with a thickness and surface condition desirable for a substrate.
The above mentioned chamfering work on the outer and inner peripheries of an annular disks as sliced and lapped is essential as a consequence of the high hardness but remarkable brittleness of a silicon single crystal as compared with aluminum-based alloys. If the chamfering work is omitted and the substrate of a silicon single crystal is finished to leave the peripheries each having a square or orthogonal cross sectional profile, namely, the sharp edges between the upper or lower surface of the disk and the peripheral surface are always under a risk of chipping which can be prevented by chamfering.
The above mentioned chamfering work of the peripheries of an annular disk of a silicon single crystal as sliced and lapped, however, involves another problem. Namely, the outer and inner peripheries of the annular disk after chamfering unavoidably have a stressed surface layer having a thickness of, for example, a few .mu.m to several tens of .mu.m affecting the mechanical strengths of the substrate as a whole eventually leading to formation of tiny cracks along the peripheries. Once formed, the cracks propagate by the high-velocity revolution of the magnetic recording disk to cause a decrease in the impact strength of the disk which consequently is subject to chipping or cracking in the manufacturing process of magnetic recording media. Such a trouble not only results in a great decrease in the productivity of acceptable products but also eventually causes damages on the apparatus in the manufacturing line and on the already finished, otherwise acceptable products due to scattering of fragments of the broken disks.